The present invention relates to a multi-beam scanning optical system in which a plurality of light beams emitted by a plurality of light source are deflected to scan on a surface to be scanned such as a circumferential surface of a photoconductive drum.
A scanning optical system to be employed in a laser printer for forming a monochrome (e.g., a black-and-white) image is provided with a laser diode, which is driven in accordance with image data. The laser beam emitted by the laser diode is collimated by a collimating lens, and is directed to a deflecting member such as a polygonal mirror. The laser beam, which is incident on light reflecting surfaces of the rotating polygonal mirror, is deflected to scan within a predetermined angular range. The scanning laser beam is incident on an f.theta. lens, refracted and converged thereby, and then is incident on an evenly charged photoconductive surface of a photoconductive drum to form a beam spot which moves along the rotational axis of the photoconductive drum (i.e., along a main scanning direction). Since the laser diode is driven in accordance with the image data, the surface of the photoconductive drum is exposed to light corresponding to the image data. While the light beam scans on the surface of the photoconductive drum, it is rotated (i.e., an auxiliary scanning is performed). Thus, a two-dimensional latent image is formed on the photoconductive surface of the photoconductive drum.
Then, toner is applied to the latent image to form a developed image, which is transferred onto a recording sheet and fixed thereon.
Recently, color laser beam printers, which is capable of forming a color image, has been developed. In the color laser printer, generally, a plurality of laser diodes are provided (which will be referred to as a multi-beam laser printer). Further, the corresponding number of f.theta. lenses, and the corresponding number of photoconductive drums are provided for forming images of respective color components (e.g., yellow, magenta, cyan and black components). The above-described, exposing and developing processes are performed for each color component, and thus formed color image (developed image) consisting of the four color components is transferred on a recording sheet and fixed.
In the multi-beam laser printer as described above, since a plurality of f.theta. lenses are employed for respective color components, the problems indicated below occur.
Firstly, if temperature differences are caused among the f.theta. lenses for respective color components, the optical characteristics of the f.theta. lenses may vary among the f.theta. lenses. In such a case, positional errors (i.e., displacements) occur among positions on the photoconductive drums, on which the beams emitted from the f.theta. lenses are incident, in the scanning direction, and therefore the formed color image may contain a color shift.
Secondly, since a plurality of f.theta. lenses are provided for the respective color components, a structure of the optical system tends to be complicated.